UT

[steveh]

Just wondering what people use to keep track of GMT on board.
Do you use a chronometer and keep a note of error from time signals
or a more modern alternative?
This is in the context of astro navigation so accurate time needed.


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[john_morris_uk]

I use a Casio digital watch. Check the time against the gps - and the rate (ie how many seconds it gains or loses over a week/day etc). Then if the electronics all go up the proverbial I can work out time to within a second or two. I keep the watch set to GMT so that my brain doesn't have to do so many calculations.

I will now admit that with 99.9% of other people the last few ocean passages have been on GPS alone. The crew know I'm bored when I get the sextant and tables out. I can argue its good to practice, but...

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[Roberto]

I measured the error of my wristwatch over a few months, with different temperatures: it advances one second a day: I check the watch with the telephone before leaving, then I hardly spend more than one month on the water...

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[Seafort]

Ut is allways available from one of the GPS sets. In addition the main cabin clock is set from the Gps on arrival (the error for this clock is known).

My watch error is also known and frequently checked against radio time signals, remember also that there is a delay to these signals on digital radio.

I guess having the GPS on might defeat the object of the astro-nav exercise but it can give a stonking EP./forums/images/icons/wink.gif

Dave.



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[steveh]

I have been told that the time on a GPS is a few seconds different to GMT. Does it make a difference?

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[snowleopard]

the error of gps time is approx 1 second. that equates to 1/4 minutes of longitude or around 1/5 of a mile in these latitudes. a lot better than your accuracy in using a sextant i'll bet!

i bought a clock with analog and digital displays. it was the only one i could find that had seconds on the display. the analogue part is accurate to a second a day, the digital part loses more than 10 seconds a day!

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[AndrewB]

For me GPS, which agrees with UT to within less than a second, provides a master clock. A little caution is necessary because if the GPS signal is lost, the time function on the unit keeps running apparently normally, but it isn't that accurate.

I use GPS time to log a check of the error on my watch every few days. With a press-button hold-time function, the watch is more convenient when taking sights. In practice a decent quartz watch is normally so accurate that really nothing else is needed.

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[tome]

There's a difference of a few seconds between UTC and GPS time (currently 13 secs) due to GPS time being 'atomic' and UTC being real-world. The offset is known as leap-seconds and increases by about 1 second every 18 months. However, the difference is transmitted by the GPS satellites and so applied in the receiver to derive UTC.

I haven't yet seen a GPS receiver which uses 'GPS' time: they all convert it to UTC. However, the update can be over 1 second late in being displayed so a bit of caution is required when using it to synchronise.

Incidentally, updating the firmware in my Garmin 128 to v2.10 reduced the delay in time output from 0.8 sec to around 0.2s!

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[tome]

Most GPS receivers have a reasonably good quartz clock with a stability of around 40ppm or better. It would take around 7 hours for it to drift one second if free-wheeling so unlikely to be a problem unless you have a fault.

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[AndrewB]

Thanks, that's quite right, but as you say no regular GPS units display uncorrected 'GPS time'. I knew there was still a tiny difference with UTC but thought it was something to do with the 'leap seconds' rather than due to delay in display.

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[tome]

The reason for the delay is that the processors used in gps receivers are run fairly hard to compute the fix and velocities plus handle all the display and data outputs, leaving certain tasks (eg time updates) as low priorities.

The cheapest OEM GPS receivers (board only - no display, case etc) cost around $50 and these have a separate pulse which comes out once per second on the second. The absolute timing accuracy of this pulse is better than half a micro-second! Unfortunately, this PPS is not brought out of the receiver on leisure products.

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[VicMallows]

tome,

What level did you update from? I've updated to 2.06, but decided that if the difference at 2.10 was really only 'French translations' it wasn't worth the bother. Maybe though there are other updates lurking in there.

Vic

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[tome]

Vic

I updated from 2.06. I did the timing check by way of experiment as I have equipment to measure it, first using 2.06 and then 2.10 after upgrading. Like you, I wasn't convinced I needed the French translations but I was doing some equipment trials and so made sure everything was at the latest release. I don't think the manufacturers ever tell us the full extent of the latest release, eg "v2.10 knocks over half a second off our shabby time delay!".

There are probably other upgrades I haven't noticed.

Regards
Tom

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[charles_reed]

Most people seem to use the GPS timer - on my old Garmin 65 and the 35 it only shows when in receipt of a valid signal.

The cheap and cheerful watch I have is rated a chronometer (which apparently means it's accurate to within a second in a year). It's accuracy goes for a ball of chalk when the battery runs out every 4th year though.

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[bluejuice]

as already said measure the time diffrence over a few months with a couple of watch top of the range casio loses 1 sec every ten days sekio watch all over the shop. this can be done on the internet have not got the site to hand, some USA goverment atomic clock in real time. best thing to do is if you have a MF/HF/SSB radio you can pick up the cordinated UT broadcast see admartly list of radio signals vol 3 (2) for frequcys. This stops you going fully clock mad (donald crowhurst) when you turn the GPS OFF

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[VicMallows]

tome,

Yes, thanks. I'll update while the set is at home over winter.

Vic

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